The present invention relates to a method of joining ceramic members which are functional parts of automobiles, etc. and its application to the manufacture of an oxygen sensor element.
To be more specific, the present invention relates to a method of joining and sealing together ceramic members under high-temperature and high-pressure conditions to provide a very durable joint, and its application to the manufacture of an oxygen sensor element.
In recent years use of ceramic members as functional parts of automobiles and internal combustion engines has positively been considered. These ceramic members have often to operate in a hot gas and are required to be gas-tight. The conventional methods of joining ceramic members can be broadly classified as: using an inorganic bonding agent; using an activated metal; using metallization; and using glass. Among others, the metallizing process has been considered as one suitable for joining members of a motor vehicle oxygen sensor element with a joint strength high enough to stand high temperatures and provide a gas-tight joint.
In the metallizing process the ceramic members to be joined are daubed with a paste of a metal powder such as molybdenum, manganese or tungsten or a combination of these metals blended with an organic substance such as methyl cellulose in an organic solvent such as butyl acetate; the ceramic members are then baked at 1300.degree.-1600.degree. C. in a reducing atmosphere to form a metallized layer thereon; the metallized layer is then plated with a metal like nickel; and thereafter silver-brazing is done between a plated layer and a metal, or between the plated layers.
In such a metallizing process, a firm bonding of the metallized layer on the ceramic surface is effected through reaction between an oxide in the metal used for metallizing and a glass contained in the ceramic, thereby forming a bonding layer.
Thus, the metal used in this process is usually one which readily oxidizes and a precious metal, say, platinum would not yield a firm bond. Therefore when brazing is done between a metallized layer obtained by using a precious metal and a metal, or between metallized layers of precious metal, the brazing material produces an excessive solid solution of the metal in the metallized layers, or even attacks the metallized layer itself, resulting in an insufficient strength of joint.
For this reason, the conventional metallizing process cannot produce a sufficiently strong joint when precious metal is used. However, where an oxygen sensor has to operate in a hot oxidizing atmosphere, its members have to be joined using a precious metal, but there is the drawback that an oxygen sensor with sufficient joint strength cannot be obtained by the conventional method.